MP increases the factor (EMS Rating) your earned EMS are multiplied with to get your Total Military Strength. Without MP, it will always be 50%. Doing MP will raise it to CAP at 100%. It will reduce slowly over time if you stop playing MP.

Say you earned the Geth on Rannoch, showing 600 EMS.

Without MP, they will only be worth 300.

Beyond that each maxed out MPChar can be converted into 75 EMS points, which are bound to your account permanently. Every Playthrough will have them right from the start.

Since the Extended Cut DLC MP is not mandatory anymore to whitness the breathing sceen as it lowered the requirements from 4000 EMS down to 3150 EMS.

You cam earn around 8300 EMS in SP, depending on file transfer and DLC's.

So the MP impact is more cosmetic, or if you always aim for the best outcome but want to skip certain content.

If you want to experience certain LOW EMS outcomes, MP will make it really hard up to the point of rendering it impossible to achieve.

To disable MP influence on the SP you simply have to play in off-line mode. ME3 won't connect to Galaxy at War server and on-line War Assets rating won't be imported. Its that easy. If you never Promoted single character in MP you can ignore that part completely. Or you can try for example Expanded Galaxy Mod which Significantly Reduces value of War Assets (my is now just shy of 300 pts instead of ~23000).

Still, with Extended Cut you have to work decently hard not to experience optimal ending.

On the other hand, SP is totally unaffected by MP in cut-scenes. Even if you have 100% galactic readiness Anderson will still say only 50% of the force is available for final push.

Rating goes down the moment you log off. Playing everyday at least couple games, galactic readiness goes down by about 2-2.5% every 12 hours or so.

I'm very much into ME3MP. It's the only MP I play which speaks volumes as I hate on-line gaming overall. Hoping that Andromeda delivers even better experience, but it will be hard to stop playing ME3. It's so close to my heart that I can't imagine at this moment in time. It's part of trilogy where you just do away with story line and shot bad guys that invaded our homes. Of course I'm most definitely averagely average on that front. Gold is max I feel comfortable (well I hate playing Geth and backstabbing Maruders). I witnessed some truly jaw dropping players who can clear Platinum maps with naked Volus. Freaking amazing.